MEMS plate switch and method of manufacture

ABSTRACT

Systems and methods for forming an electrostatic MEMS plate switch include forming a deformable plate on a first substrate, forming the electrical contacts on a second substrate, and coupling the two substrates using a hermetic seal. The deformable plate may have a flexible shunt bar which has one end coupled to the deformable plate, and the other end coupled to a contact on the second substrate. Upon activating the switch, the deformable plate urges the shunt bar against a second contact formed in the second substrate, thereby closing the switch. The hermetic seal may be a gold/indium alloy, formed by heating a layer of indium plated over a layer of gold. Electrical access to the electrostatic MEMS switch may be made by forming vias through the thickness of the second substrate.

CROSS REFERENCE TO RELATED APPLICATION

This U.S. patent application is a continuation-in-part of U.S. patentapplication Ser. No. 11/797,924, which is incorporated by referenceherein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Portions of the present invention were made with U.S. Government supportunder NSF SBIR Grant No. 0637474. The government may have certain rightsin this invention.

STATEMENT REGARDING MICROFICHE APPENDIX

Not applicable.

BACKGROUND

This invention relates to a microelectromechanical systems (MEMS) switchdevice, and its method of manufacture.

Microelectromechanical systems are devices often having moveablecomponents which are manufactured using lithographic fabricationprocesses developed for producing semiconductor electronic devices.Because the manufacturing processes are lithographic, MEMS devices maybe made in very small sizes, and in large quantities. MEMS techniqueshave been used to manufacture a wide variety of sensors and actuators,such as accelerometers and electrostatic cantilevers.

MEMS techniques have also been used to manufacture electrical relays orswitches of small size, often using an electrostatic actuation means toactivate the switch. MEMS devices often make use of silicon-on-insulator(SOI) wafers, which are a relatively thick silicon “handle” wafer with athin silicon dioxide insulating layer, followed by a relatively thinsilicon “device” layer. In the MEMS devices, a thin cantilevered beam ofsilicon may be etched into the silicon device layer, and a cavity iscreated adjacent to the thin beam, typically by etching the thin silicondioxide layer below it to allow for the electrostatic deflection of thebeam. Electrodes provided above or below the beam may provide thevoltage potential which produces the attractive (or repulsive) force tothe cantilevered beam, causing it to deflect within the cavity.

One known embodiment of such an electrostatic relay is disclosed in U.S.Pat. No. 6,486,425 to Seki. The electrostatic relay described in thispatent includes a fixed substrate having a fixed electrode on its uppersurface and a moveable substrate having a moveable electrode on itslower surface. Upon applying a voltage between the moveable electrodeand the fixed electrode, the moveable substrate is attracted to thefixed substrate such that an electrode provided on the moveablesubstrate contacts another electrode provided on the fixed substrate toclose the microrelay.

However, to fabricate the microrelay described in U.S. Pat. No.6,486,425, the upper substrate must be moveable, so that the uppersubstrate must be thin enough such that the electrostatic force maycause it to deflect. The moveable substrate is formed from asilicon-on-insulator (SOI) wafer, wherein the moveable feature is formedin the silicon device layer, and the SOI wafer is then adhered to thefixed substrate. The silicon handle wafer and silicon dioxide insulatinglayer are then removed from the SOI wafer, leaving only the thin silicondevice layer which forms the moveable structure.

SUMMARY

Because the top substrate of the microrelay described in the '425 patentmust necessarily be thin enough to be moveable, it is also delicate andsusceptible to damage from contact during or after fabrication.

The systems and methods described here form an electrostatic MEMS plateswitch using dual substrates, a first, lower substrate on which to forma deformable plate with at least one electrical shunt bar to provide anelectrical connection between the contacts of a switch. These contactsmay be formed on a second, upper substrate. After forming thesestructures, the two substrates are bonded together to form the switch.It should be understood that the designation of “upper” and “lower” isarbitrary, that is, the deformable plate may also be formed on an uppersubstrate and the contacts may be formed on a lower substrate.

The electrostatic MEMS plate switch design may have a number ofadvantages over cantilevered switch designs. One advantage may be thatthe plate may be lowered onto an adjacent electrode while remainingparallel to that substrate, so that there is less tendency for theelectrostatic plates to arc at their position of closest approach. Also,multiple sets of switch contacts may be placed on a single deformableplate, whereas with the cantilevered design, only the area at the distalend of the cantilevered beam is generally appropriate for the placementof the switch contacts.

Accordingly, in the systems and methods described here, the deformableplate is attached to the first SOI substrate by one or more narrowspring beams formed in the device layer of the SOI substrate. Thesespring beams remain fixed at their proximal ends to the silicon dioxidedielectric layer and handle layer of the SOI substrate. A portion of thesilicon dioxide layer adjacent to the deformable plate may be etched torelease the plate, however, a silicon dioxide attachment point remainswhich couples the spring beams supporting the deformable plate to thesilicon handle layer. The silicon dioxide layer therefore provides theanchor point for attachment of the deformable plate to the first, lowerSOI substrate from which it was made. Because the remainder of therigid, SOI wafer remains intact, it may provide protection for theswitch against inadvertent contact and shock.

Because the rigid SOI wafer remains intact, it may also be hermeticallybonded to a second, upper substrate at the end of the fabricationprocess. By forming the hermetic seal, the switch may enclose aparticular gas environment which may be chosen to suit a particularpurpose, such as increasing the breakdown voltage or altering thethermal properties of the gas environment within the switch.Alternatively, the environment surrounding the plate switch may bevacuum, which may increase the switching speed of the plate switch bydecreasing viscous squeeze film damping which may arise in a gasenvironment. The hermetic seal may also protect the electrostatic MEMSswitch from ambient dust and debris, which may otherwise interfere withthe proper functioning of the device.

In one exemplary embodiment, the deformable plate formed on the firstsubstrate may carry one or more shunt bars, placed at or near the nodallines for a vibrational mode of the deformable plate. Points along theselines remain relatively stationary, even though the deformable plate maystill be vibrating in a vibrational mode.

In another exemplary embodiment, referred to herein as the singlecontact MEMS plate switch embodiment, the deformable plate may have ashunt bar anchored to the moving, deformable plate to form a movingcontact on the shunt bar, whereas the other end of the shunt bar iscoupled to a stationary contact rigidly attached to a member other thanthe deformable plate. The shunt bar may flex between these contactpoints. When the switch is activated, the deformable plate may press themoving end of the shunt bar against a second contact thereby closing theswitch. The first contact and second contact may be affixed to a secondsubstrate. Thus, an electrical connection between an input line and anoutput line may be made with only one contact or junction. This mayfurther reduce the contact resistance of the switch, because there isonly a single junction between the input and output lines.

In order to allow the shunt bar to flex out of the plane of thedeformable plate, voids may be formed in the deformable plate around ornear the contacts. By removing material near the stationary contact inparticular, the shunt bar is allowed to flex out of the plane of thedeformable plate, to open or close the switch. One or more voids may beformed around or near each contact. Multiple voids, if present, may beseparated by a thin isthmus of material remaining of the deformableplate. The isthmus may be coupled to the moving contact, and may beconfigured to move either laterally and/or rotationally as the switch isclosed. Movement of the isthmus may therefore provide some scrubbing ofthe contact surfaces, which may further reduce the contact resistance ofthe switch, by clearing contamination and debris.

In one exemplary embodiment, a method for manufacturing the MEMS plateswitch may include forming a first plate suspended adjacent to a firstsubstrate by least one spring beam, coupling at least one shunt bar tothe first plate at one location on the at least one shunt bar, couplingthe at least one shunt bar to a first contact, the first contact not onthe first plate, at another location on the at least one shunt bar, andconfiguring the first plate to activate the switch by pressing the atleast one shunt bar against a second contact. The switch formed by thismethod may include a first plate suspended adjacent to a first substrateand coupled to the first substrate by at least one spring beam, at leastone shunt bar coupled to the first plate at one location on the at leastone shunt bar, and coupled to a first contact at another location on theat least one shunt bar, the first contact not on the first plate,wherein the first plate is configured to activate the switch by pressingthe at least one shunt bar against a second contact.

In one exemplary embodiment, the deformable plate is coupled to thefirst, SOI substrate by four flexible spring beams which are anchored tothe dielectric layer of the SOI substrate at the proximal end of eachspring beam. The other end of the spring beams may be contiguous withthe deformable plate. The spring beams may include a bend of at leastninety degrees, so that each spring beam on one side of the deformableplate extends in an opposite direction from the other. This embodimentmay be referred to as the symmetric embodiment, as the two spring beamson each side of the deformable plate may have the same shapes andorientations as the two spring beams on the other side of the deformableplate. In another “asymmetric” embodiment, the spring beams on one sideof the deformable plate may extend in one direction, and the springbeams on the other side of the deformable plate may extend in theopposite direction. The asymmetric embodiment may therefore be capableof twisting during vibration, which may provide additional scrubbingaction to the deformable plate. The scrubbing action may clearcontamination and debris, thus reducing the contact resistance betweenthe shunt bar on the deformable plate and the contact between the shuntbar and the input contact.

In one exemplary embodiment, etch release holes may be placed betweenthe nodal lines of the deformable plate, so that the deformable platemay be made more flexible in critical regions. The etch release holesmay thereby encourage vibration in a particular vibrational mode overvibrations in other modes. In other exemplary embodiments, the etchrelease holes may be placed uniformly about the deformable plate in aclose-packed hexagonal array. This arrangement may reduce the mass ofthe deformable plate, and allow ambient gas to flow through the etchrelease holes and thus reducing squeeze film damping and increasing theswitching speed of the deformable plate.

A hermetic seal may enclose the dual substrate MEMS plate switch. Thehermetic seal may be made by forming a metal bond between thesubstrates, the bond being an alloy of gold and indium, AuIn_(x), wherex is about 2. The alloy may be formed by melting a layer of indiumdeposited over a layer of gold. The hermetic seal is therefore alsoconductive, and may provide electrical access to the deformable plate,for example. The hermetic seal may be particularly important forswitching applications involving relatively high voltage signals,wherein an insulating gas may be needed to prevent electrical breakdownof the environment between the high voltage electrodes. In such cases,the insulating gas, or vacuum, may need to be sealed hermetically tocreate an environment for the MEMS switch which can withstand highervoltages without breaking down or alter the thermal properties of theswitch, without allowing the gas to leak out of, or into, the MEMSswitch seal.

In another exemplary embodiment, electrical access to the switch may begained using through hole vias formed through the second substrate. Byproviding electrical access through the second substrate, the hermeticseal may not be compromised by the presence of electrical leads beingrouted under the bond line of the seal.

The systems and methods described herein may be appropriate for thefabrication of an RF electrostatic MEMS plate switch which is capable ofoperating in the range of DC to at least 10 GHz, and having an actuationvoltage in the range of 35-50V.

These and other features and advantages are described in, or areapparent from, the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Various exemplary details are described with reference to theaccompanying drawings, which however, should not be taken to limit theinvention to the specific embodiments shown but are for explanation andunderstanding only.

FIG. 1 is a cross sectional view of an exemplary dual substrateelectrostatic MEMS plate switch;

FIG. 2 is a greyscale image of the third vibrational mode of adeformable plate such as that used in the plate switch of FIG. 1;

FIG. 3 is a plan view of one exemplary embodiment of the deformableplate of the dual substrate electrostatic MEMS plate switch of FIG. 1,showing the locations of the two shunt bars along the nodal lines of thedeformable plate;

FIG. 4 is a greyscale image of the deformable plate in the thirdvibrational mode upon making contact with electrodes located below theshunt bars;

FIG. 5 is a plan view of a second exemplary embodiment of a deformableplate usable in the dual substrate MEMS plate switch of FIG. 1;

FIG. 6 is a plan view of a design for a third exemplary embodiment of adeformable plate usable in the dual substrate MEMS plate switch of FIG.1;

FIG. 7 is a diagram showing a first step in an exemplary method ofmanufacturing the first plate substrate of the dual substrate MEMS plateswitch of FIG. 1, using the deformable plate of FIG. 6;

FIG. 8 is a diagram showing a second step in an exemplary method ofmanufacturing the first plate substrate of the dual substrate MEMS plateswitch of FIG. 1, using the deformable plate of FIG. 6;

FIG. 9 is a diagram showing a third step in an exemplary method ofmanufacturing the first plate substrate of the dual substrate MEMS plateswitch of FIG. 1, using the deformable plate of FIG. 6;

FIG. 10 is a diagram showing a first step in an exemplary method ofmanufacturing the second via substrate of the dual substrate MEMS plateswitch of FIG. 1;

FIG. 11 is a diagram showing a second step in an exemplary method ofmanufacturing the second via substrate of the dual substrate MEMS plateswitch of FIG. 1;

FIG. 12 is a diagram showing a third step in an exemplary method ofmanufacturing the second via substrate of the dual substrate MEMS plateswitch of FIG. 1;

FIG. 13 is a diagram showing a greater detail of the lower electrodeformed on the second via substrate of the dual substrate MEMS plateswitch of FIG. 1;

FIG. 14 is a diagram showing the bonding pad design formed on thebackside of the dual substrate MEMS plate switch of FIG. 1;

FIG. 15 is a diagram of the completed dual substrate MEMS plate switchof FIG. 1, with an indication of the cross section shown in FIG. 16;

FIG. 16 is a cross sectional view of the dual substrate MEMS plateswitch along the cross section indicated in FIG. 15;

FIG. 17 is a plan view of the deformable plate of an electrostatic MEMSplate switch with a single contact;

FIG. 18 is a perspective view of the deformable plate shown in FIG. 17,showing the adjacent contacts formed in the second substrate;

FIG. 19 is a three-dimensional perspective view of the deformable plateshown in FIG. 17 when deflected;

FIG. 20 is a cross sectional view of the single contact electrostaticMEMS plate switch;

FIG. 21 is a cross sectional view of the plate substrate of the singlecontact electrostatic MEMS plate switch in a first step of fabrication;

FIG. 22 is a cross sectional view of the plate substrate of the singlecontact electrostatic MEMS plate switch in a second step of fabrication;

FIG. 23 is a cross sectional view of the plate substrate of the singlecontact electrostatic MEMS plate switch in a third step of fabrication;

FIG. 24 is a cross sectional view of the plate substrate of the singlecontact MEMS plate switch in a first step of fabrication aligned withits via substrate before bonding;

FIG. 25 is a cross sectional view of the plate substrate of the singlecontact electrostatic MEMS plate switch after bonding to its viasubstrate; and

FIG. 26 is a cross sectional view of the plate substrate of the singlecontact electrostatic MEMS plate switch in alternative embodiment usingan engineered substrate.

FIG. 27 is a simplified diagram of the single contact MEMS plate switch,illustrating the basic shape and functioning of the cut-out, voidportion surrounding the stationary contact;

FIG. 28 is a simplified diagram of an additional embodiment of thesingle contact MEMS plate switch, illustrating the shape and functioningof the cut-out, void portion surrounding the stationary contact, whichprovides lateral movement of the contact surfaces; and

FIG. 29 is a simplified diagram of an additional embodiment of thesingle contact MEMS plate switch, illustrating the basic shape andfunctioning of the cut-out, void portion surrounding the stationarycontact, which provides rotational movement of the contact surfaces.

DETAILED DESCRIPTION

In the systems and methods described here, an electrostatic MEMS switchis fabricated on two substrates. A deformable plate carrying at leastone shunt bar is formed on the first substrate, and the electricalcontacts of the switch, which will be connected via the shunt bar on thedeformable plate when the switch is closed, are formed on the othersubstrate. The words “shunt bar” as used herein should be understood tomean any shape of conductive material which is used to transmitelectrical signals from one point to another. In one exemplaryembodiment, the shunt bar is a relatively long but thin layer ofconductive material deposited on the deformable plate. The twosubstrates may then be sealed hermetically by a gold-indium seal.Electrical access to the switch may be afforded by a set of through holevias, which extend through the thickness of the second substrate.Although the systems and methods are described as forming the deformableplate first on the first substrate followed by the electrical contactson the second substrate, it should be understood that this embodiment isexemplary only, and that the electrical contacts may be formed first, orin parallel with, the formation of the deformable plate.

FIG. 1 is a cross sectional view of the dual substrate electrostaticMEMS plate switch 100 fabricated on two substrates, a plate substrate1000 and a via substrate 2000. The plate substrate 1000 may be an SOIwafer, and the via substrate may be a silicon wafer, for example. Theelectrostatic MEMS switch 100 may include a plate 1300 bearing at leastone shunt bar 1100. The plate may be deformable, meaning that it issufficiently thin compared to its length or its width to be deflectedwhen a force is applied, and may vibrate in response to an impact. Forexample, a deformable plate may deflect by at least about 10 nm at itscenter by a force of about 1 μNewton applied at the center, andsufficiently elastic to support vibration in a plurality of vibrationalmodes. The deformable plate 1300 may be suspended above the handle layer1030 of an SOI substrate by four spring beams (not shown in FIG. 1),which may themselves be affixed to the handle layer 1030 by anchorpoints formed from the dielectric layer 1020 of the SOI plate substrate1000. As used herein, the term “spring beam” should be understood tomean a beam of flexible material affixed to a substrate at a proximalend, and formed in substantially one plane, but configured to move andprovide a restoring force in a direction substantially perpendicular tothat plane. The deformable plate may carry at least one conductive shuntbar which operates to close the electrostatic MEMS switch 100, asdescribed below.

In one embodiment, each shunt bar is designed to span two contactpoints, 2110 and 2120, which are through wafer vias formed in the viasubstrate 2000, and covered by a layer of contact material 2112 and2122, respectively. The deformable plate may be actuatedelectrostatically by an adjacent electrostatic electrode 2300, which maybe disposed directly above (or below) the deformable plate 1300, and maybe fabricated on the via substrate 2000. The deformable plate 1300itself may form one plate of a parallel plate capacitor, with theelectrostatic electrode 2300 forming the other plate. When adifferential voltage is placed on the deformable plate 1300 relative tothe adjacent electrostatic electrode 2300, the deformable plate is drawntoward the adjacent electrostatic electrode 2300. The action moves theshunt bar 1100 into a position where it contacts the contact points 2110and 2120, thereby closing an electrical circuit. Although the embodimentillustrated in FIG. 1 shows the plate formed on the lower substrate andthe vias and contacts formed on the upper substrate, it should beunderstood that the designation “upper” and “lower” is arbitrary. Thedeformable plate may be formed on either the upper substrate or lowersubstrate, and the vias and contacts formed on the other substrate.However, for the purposes of the description which follows, theembodiment shown in FIG. 1 is presented as an example, wherein the plateis formed on the lower substrate and is pulled upward by the adjacentelectrode formed on the upper substrate.

FIG. 2 is a greyscale image of a thin, deformable plate in a vibrationalmode. The image was generated by a finite element model, using platedimensions of 200 μm width by 300 μm length by 5 μm thickness. Thedeformable plate is supported by four spring beams 10 μm wide and 5 μmthick, extending from two sides of the deformable plate. According tothe model, a first vibrational mode with a frequency of 73 kHz may besimply the movement of the entire plate, substantially undeflected,toward and away from the surface to which it is attached by the springbeams. A second vibrational mode with a frequency of 171 kHz occurs whenthe deformable plate twists about its long axis, by bending at thejoints between the deformable plate and the spring beams.

However, another vibrational mode exists as illustrated by FIG. 2, whichis encouraged by the proper placement of the spring beams. The springbeams are placed at approximately the location of the node lines forthis vibrational mode. By placing the spring beams at these points, theplate may vibrate with relatively little deflection of the spring beams.The frequency associated with this mode is at about 294 kHz.

As a result, the deformable plate vibrates substantially in the thirdvibrational mode, with the node lines of the vibration locatedsubstantially at the locations of the supporting spring beams. Thesenode lines indicate points on the deformable plate which remainrelatively stationary, compared to the ends and central region which aredeflected during the vibration. The existence of these node linesindicate advantageous locations for the placement of electrodes for aswitch, because even when the plate is vibrating, there is relativelylittle deflection of the plate along the node lines. Accordingly, if ashunt bar is placed at the node lines, the shunt bar may provideelectrical conductivity between two electrodes located beneath the shuntbar, even if the plate continues to vibrate.

FIG. 3 is a plan view of a first exemplary embodiment of a deformableplate useable in the plate switch of FIG. 1. The plate is supported byfour spring beams 1330, which are attached to the underlying substrateat their proximal ends 1335. One pair of the four spring beams may bedisposed on one side of the deformable plate, and another pair of thefour spring beams may be disposed on an opposite side of the deformableplate. Each spring beam may have a segment extending from the deformableplate which is coupled to an adjoining segment by a bend. The choice ofangle for this bend may affect the kinematics of the deformable plate1300.

In the embodiment shown in FIG. 3, the spring beams include a ninetydegree bend, such that each spring beam on each side of the deformableplate 1300 extends in an opposite direction to the adjacent spring beam.This embodiment may be referred to as the symmetric embodiment, as theorientation of the deformable plate and spring beams is symmetric withrespect to reflection across either a longitudinal or latitudinal axisof the deformable plate, wherein the longitudinal or latitudinal axis isdefined as horizontal or vertical line, respectively, passing throughthe center of the deformable plate. It should be understood that thisembodiment is exemplary only, and that the spring beams may bend withother angles, for example, twenty or thirty degrees, rather than ninetyas shown in FIG. 3.

The two nodal lines for the third vibrational mode are shown in FIG. 3.One of two shunt bars 1110 and 1120 may be placed across each nodalline. The shunt bars 1110 and 1120 may be electrically isolated from thedeformable plate by a layer of dielectric 1210 and 1220, respectively.Additional dielectric standoffs 1230 may be formed at the corners ofdeformable plate 1300, to prevent deformable plate 1300 from contactingthe adjacent electrostatic electrode 2300 at the corners of deformableplate 1300, when actuated by the adjacent electrostatic electrode 2300.The shunt bars 1110 and 1120 may be dimensioned appropriately to spanthe distance between two underlying electrical contacts, 2110 and 2120under shunt bar 1110, and contacts 2210 and 2220 under shunt bar 1120.The deformable plate 1300 is actuated when a voltage differential isapplied to an adjacent electrode, which pulls the deformable plate 1300toward the adjacent electrode. If the deformable plate 1300 vibrates asa result of actuation, it is likely to vibrate in the third vibrationalmode shown in FIG. 2. Accordingly, the shunt bars 1110 and 1120 areplaced advantageously at the nodal lines of this vibrational mode.

The tendency of deformable plate 1300 to vibrate in the thirdvibrational mode may be enhanced by placing etch release holes 1320along the latitudinal axis passing through the center of the deformableplate, between the nodal lines, as shown in FIG. 3. These etch releaseholes are used to assist the liquid etchant in accessing the farrecessed regions beneath the deformable plate, to remove the dielectriclayer beneath the plate, as described further in the exemplarymanufacturing process set forth below. By placing these etch releaseholes appropriately, the deformable plate 1300 may be made more flexiblein certain regions, such as along the latitudinal axis, such that theplate is encouraged to vibrate in a mode such that the maximumdeflection occurs where the plate is more flexible. For example, toencourage the vibration as shown in FIG. 2, the plate may be made moreflexible along its latitudinal axis, in order to accommodate theregion's undergoing the maximum deflection, by placing etch releaseholes 1320 along this latitudinal axis.

In another alternative embodiment, the etch release holes are disposedin a close-packed hexagonal array over the entire surface of thedeformable plate 1300. Such an embodiment may be advantageous in thatthe mass of the deformable plate is reduced, and multiple pathways areprovided for the flow of the ambient gas to either side of thedeformable plate. Both of these effects may improve the switching speedof the device by reducing the inertia of the deformable plate 1300 andreducing the effects of squeeze film damping.

FIG. 4 is a greyscale image of the deformable plate shown in FIG. 3,after actuation by an adjacent electrode, calculated by a finite elementmodel. As shown in FIG. 4, the deformable plate is pulled down towardthe adjacent electrode, which in this case is located beneath thedeformable plate 1300. The lowest areas of the deformed plate are in thevicinity of the contacts, also located beneath the deformable plate1300. When the deformable plate is deflected as shown in FIG. 4, theshunt bars affixed to the deformable plate are lowered onto theunderlying contacts, thus providing a conductive path between thecontacts and closing the electrostatic MEMS switch 100. Any residualvibration in the deformable plate is primarily in the third vibrationalmode, depicted in FIG. 2. Thus, for shunt bars placed as shown in FIG.3, the residual vibration does not substantially affect the ability ofswitch electrostatic MEMS switch 100 to close the conductive pathbetween the contacts.

Also as shown in FIG. 4, the corners of deformable plate 1300 tend to bedrawn towards the adjacent actuation electrode. The dielectric standoffs1230 may prevent the touching of corners of the deformable plate 1300 tothe adjacent electrode, thus shorting the actuation voltage. Theactuation voltage in this simulation is about 40 volts, and the size ofthe deformable plate is about 200 μm by 300 μm. This actuation voltageproduces a deflection of at least about 0.6 μm in the deformable plate.This deflection is about ⅓ of the overall separation between the shuntbars and the electrodes, which may nominally be about 2.5 μm, and issufficient to cause snap-down of the deformable plate onto theunderlying contacts. Although as shown in FIG. 4, the maximum deflectionis near the center of the plate, this effect may be altered by disposingthe spring beams at an angle shallower than ninety degrees. Such anarrangement may result in a more consistent force being applied betweenthe shunt bar and each of the underlying contacts.

FIG. 5 is a plan view of a second exemplary embodiment of the deformableplate 1300′. Deformable plate 1300′ may differ from deformable plate1300 by the placement and orientation of the four spring beams whichsupport the deformable plate 1300′. A first set of spring beams 1332′are coupled to one side of the deformable plate 1300′, and a second setof spring beams 1330′ are coupled to the other side of deformable plate1300′. However, in contrast to the spring beams 1330 of deformable plate1300, spring beams 1330′ extend in an opposite direction to spring beams1332′ of deformable plate 1300′, after the bend in spring beams 1330′and 1332′. This may allow deformable plate 1300′ to twist and translatesomewhat in the plane of the deformable plate 1300′, upon actuation byapplying a differential voltage between deformable plate 1300′ and anadjacent electrode, because of the flexibility of the bend between thebeam segments. This twisting action may allow some lateral movement ofshunt bars 1100 over contacts 2110 and 2120, thereby scrubbing thesurfaces of the contacts to an extent. This scrubbing action may removecontamination and debris from the contact surfaces, thereby allowingimproved contact and lower contact resistance.

The embodiment shown in FIG. 5 may be referred to as the anti-symmetricembodiment, because the spring beams 1330′ extend from the beam in anopposite direction compared to spring beams 1332′. In other words, inthe anti-symmetric embodiment 1300′, the beam springs disposed on oneside of the deformable plate are anti-symmetric with respect to thebeams springs disposed on the opposite side of the deformable plate.Thus, when deformable plate 1300′ is reflected across a longitudinal orlatitudinal axis, the spring beams extend in an opposite direction fromthe bend. It should be understood that although a ninety-degree bend isillustrated in FIG. 5, the bend may have angles other thanninety-degrees, for example, for example, twenty or thirty degrees.

As shown in FIGS. 3 and 5, the deformable plate 1300 may have two shuntbars 1100 placed upon dielectric isolation layers 1100. Each shunt barmay close a respective set of contacts. For example, shunt bar 1110 inFIG. 3 may close one set of contacts 2210 and 2220, whereas shunt bar1120 may close a second set of contacts 2110 and 2120. Therefore, eachdual substrate MEMS plate switch may actually have two sets of switchcontacts disposed in parallel with one another. The dual substrate MEMSplate switch may therefore still operate if one set of switch contactsfails. Furthermore, the overall switch resistance is only one-half ofthe switch resistance that would exist with a single set of switchcontacts, because the two switches are arranged in parallel with oneanother.

FIG. 6 is a plan view of a layout of deformable plate 1300, showingadditional detail of the embodiment. In particular, spring beams 1330are formed with cutouts 1350 which penetrate the deformable plate 1300.The deformable plate may also have relieved areas 1340 formed near thelocations of the shunt bars 1100. Both the cutouts 1350 and the relievedareas 1340 give the deformable plate additional flexibility in the areaof the junction with the spring beams 1330. This may help decouple themotion of the plate 1300 from the deflection of the spring beams 1330.These features 1350 and 1340 may also help the deformable plate 1300 toclose the switch effectively, in the event that the contacts 2210 and2220 are recessed somewhat from the surface of the via substrate 2000,by giving the deformable plate 1300 additional flexibility in the regionaround the shunt bars 1200.

As shown in FIG. 6, deformable plate 1300 may have a length of about 300μm and a width of about 200 μm. The separation d1 between the springbeams may be about 130 μm. The lengths of each segment d2 and d3 of thespring beams 1330 may be about 100 μm, so that the total length of thespring beams 1330 is about 200 μm. The lengths d4 of the cutouts 1350may be about 50 μm, or about half the length of the beam segment d3. Thewidth of the spring beam 1330 may be about 12 μm. The distance betweenthe relieved areas 1340 may also be about 100 μm. The dimensions of theshunt bars 1100 may be about 40 μm width and about 60 μm length. Thediameter of the via contacts 2110 and 2120 may be about 30 μm to about50 μm. It should be understood that these dimensions are exemplary only,and that other dimensions and designs may be chosen depending on therequirements of the application.

Since the deformable plate 1300 may be made from the device layer 1010of the SOI plate substrate 1000, it may be made highly resistive, of theorder 20 ohm-cm. This resistivity may be sufficient to carry theactuation voltage of about 40 volts, but may too high to support thehigher frequency alternating current voltages associated with the firstvibrational mode at about 72 kHz. Accordingly, the resistivity mayelectrically dampen capacitive plate vibrations, especially thewhole-body first mode plate vibration.

The electrostatic plate switch design illustrated in FIG. 6 may have anumber of advantages over cantilevered switch designs, wherein theswitch contacts are disposed at the end of a cantilevered beam. Forexample, as described above, multiple sets of switch contacts may beprovided along a deformable plate, thereby reducing the overall switchresistance and therefore the loss across the switch. The multiple switchcontacts also provide redundancy, such that the switch may still beuseable even if one set of switch contacts fails. These design optionsare generally not available in a cantilevered switch design, because thecontacts are necessarily placed at the distal end of the cantileveredbeam.

In addition, the electrostatic MEMS plate switch 100 may be made morecompact than a cantilevered switch, because a long length ofcantilevered beam is not required to have a sufficiently flexible memberto actuate with modest voltages. For example, the plate designillustrated in FIG. 6 may be actuated with only 40 volts, because thespring beams 1330 which support the deformable plate may be maderelatively flexible, without impacting the spacing between theelectrical contacts 2110 and 2120.

Because the restoring force of the switch is determined by the springbeam 1330 geometry, rather than the plate 1300 geometry, modificationsmay be made to the plate 1300 design without affecting the kinematics ofthe spring beams 1330. For example, as mentioned above, a plurality ofetch release holes 1310 may be formed in the deformable plate 1300,without affecting the stiffness of the restoring spring beams 1330.These release holes 1310 may allow air or gas to transit readily fromone side of the deformable plate 1300 to the other side, therebyreducing the effects of squeeze film damping, which would otherwisereduce the speed of the device. These etch release holes 1310 may alsoreduce the mass of the deformable plate 1300, also improving itsswitching speed, without affecting the restoring force acting on thedeformable plate 1300 through the spring beams 1330.

By placing the shunt bars near the nodal lines of a vibrational mode,the switching speed may be improved because the shunt contact interfereswith vibratory motion in other modes. This effectively damps thevibrations in other modes. By placing the shunt bars at the nodal linesof a vibrational mode, the movement of the shunt bar is minimal, even ifthe plate is still vibrating in this mode. Therefore, although thedeformable plate may be made exceptionally light and fast because of itssmall size and plurality of etch release holes, it vibrates onlyminimally because of its damping attributes. Accordingly, theelectrostatic MEMS plate switch illustrated in FIG. 6 may be used in avacuum environment, which is often not possible because in a vacuum,vibrations are no longer damped by viscous air motion around the movingmember of the switch.

Because through wafer vias are used to route the signal to and from thedual substrate electrostatic MEMS plate switch 100, the electrostaticMEMS switch 100 may be particularly suited to handling high frequency,RF signals. Without the through wafer vias, the signal would have to berouted along the surface of the second via substrate 2000, and under thehermetic bond line. However, because the hermetic bond line is metallicand grounded, this allows substantial capacitive coupling to occurbetween the surface-routed signal lines and the ground plane of thedevice, which lies directly adjacent to, and narrowly separated from thesignal lines in the bonding area. The through wafer vias allow thisgeometry to be avoided, thus reducing capacitive coupling andsubstantially improving the bandwidth of the device. The through wafervias may also act as heat sinks, leading the heat generated in theswitch to be directed quickly to the opposite side of the wafer and tothe large bonding pads 2115 and 2125 on the backside of the device fordissipation.

FIGS. 7-15 depict steps in an exemplary method for manufacturing thedual substrate electrostatic MEMS plate switch 100. The steps aredivided into three sections: those steps depicted in FIGS. 7-9pertaining to the preparation of features on the plate substrate 1000;those steps depicted in FIGS. 10-12 pertaining to the preparation offeatures on the via substrate 2000; those steps depicted in FIGS. 14-15pertaining to the bonding to the plate substrate 1000 to the viasubstrate 2000, and formation of bond pads on the backside of the viasubstrate, to complete the device. FIG. 16 shows a cross section of thecompleted device shown in plan view in FIG. 15.

FIG. 7 depicts a first step in an exemplary method for manufacturing thefeatures on the plate substrate 1000 shown in FIG. 1. The platesubstrate 1000 may be a silicon-on-insulator substrate including a 5 μmthick device layer 1010, a 2 μm thick buried dielectric layer 1020, anda 500 μm thick handle layer 1030. In one exemplary embodiment, theburied dielectric layer may be a layer of silicon dioxide, and the stepsdescribed below are appropriate for this embodiment. The first step mayinclude the formation of etch release holes 1310 in the device layer1010 of the SOI plate substrate 1000. These holes 1310 may be formed bydepositing and patterning photoresist in the appropriate areas, and dryetching the release holes through the device layer 1010, using thedielectric layer 1020 as an etch stop. These release holes 1310 may be,for example, about 2 μm to about 10 μm in diameter. If the release holesare distributed over the surface of the deformable plate to reduce themass of the plate and improve mode coupling, they may be arranged in ahexagonal, close-packed array with diameters of 2 μm and spaced 3 μmapart. Deep reactive ion etching (DRIE) may be performed to etch therelease holes using, for example, an etching tool manufactured bySurface Technology Systems of Newport, UK. Such a tool may be used forthis and later DRIE steps, described below.

After etching the release holes 1310 in the device layer 1030, a thinmultilayer of 15 nm chromium (Cr) and 100 nm nickel (Ni) may besputtered onto the backside of the plate substrate 1000, for use as aplating base for the plating of a thicker layer of protective material,such as copper (Cu) or nickel (Ni). This protective layer of copper ornickel may protect the native oxide 1040 existing on the backside of thehandle layer 1030 of the SOI substrate during the hydrofluoric acid etchto follow. The protective layer of copper or nickel may be about 4 μmthick, and may also minimize the wafer bow during further processing.

The dielectric layer 1020 may then be etched away beneath and around theetch release holes 1310, using a hydrofluoric acid liquid etchant, forexample. The liquid etch may remove the silicon dioxide dielectric layer1020 in all areas where the deformable plate 1300 is to be formed. Theliquid etch may be timed, to avoid etching areas that are required toaffix the spring beams 1330 of the deformable plate1300, which will beformed later, to the handle layer 1030. Additional details as to the dryand liquid etching procedure used in this method may be found in U.S.patent application Ser. No. 11/359,558, incorporated by reference in itsentirety.

The next step in the exemplary method is the formation of the dielectricpads 1200, 1210, and 1220, and dielectric standoffs 1230 as depicted inFIG. 3. Pad structures 1200, 1210 and 1220 form an electrical isolationbarrier between the shunt bar 1100 and the deformable plate 1300,whereas standoffs 1230 form a dielectric barrier preventing the cornersof the deformable plate 1300 from touching the adjacent actuationelectrode 2300. The deformable plate 1300 and adjacent actuationelectrode 2300 form the two plates of a parallel plate capacitor, suchthat a force exists between the plates when a differential voltage isapplied to them, drawing the deformable plate 1300 towards the adjacentactuation electrode 2300.

The dielectric structures 1200, 1210, 1220 and 1230 may be silicondioxide, which may be sputter-deposited over the surface of the devicelayer 1010 of the SOI plate substrate 1000. The silicon dioxide layermay be deposited to a depth of, for example, about 300 nm. The 300 runlayer of silicon dioxide may then be covered with photoresist which isthen patterned. The silicon dioxide layer may then be etched to formstructures 1200, 1210, 1220 and 1230. The photoresist may then beremoved from the surface of the device layer 1010 of the SOI platesubstrate 1000. Because the photoresist patterning techniques are wellknown in the art, they are not explicitly depicted or described infurther detail.

FIG. 8 depicts a second step in the preparation of the SOI platesubstrate 1000. In the second step, a conductive material is depositedand patterned to form the shunt bar 1200 and a portion of what will formthe hermetic seal. The hermetic seal may include a metal alloy formedfrom melting a first metal into a second metal, and forming an alloy ofthe two metals which blocks the transmission of gases. In preparation offorming the hermetic seal, a perimeter of the first metal material 1400may be formed around the deformable plate 1300. The conductive materialmay actually be a multilayer comprising first a thin layer of chromium(Cr) for adhesion to the silicon and/or silicon dioxide surfaces. The Crlayer may be from about 5 nm to about 20 nm in thickness. The Cr layermay be followed by a thicker layer about 300 nm to about 700 nm of gold(Au), as the conductive metallization layer. Preferably, the Cr layer isabout 15 nm thick, and the gold layer is about 600 nm thick. Anotherthin layer of molybdenum may also be used between the chromium and thegold to prevent diffusion of the chromium into the gold, which mightotherwise raise the resistivity of the gold.

Each of the Cr and Au layers may be sputter-deposited using, forexample, an ion beam deposition chamber (IBD). The conductive materialmay be deposited in the region corresponding to the shunt bar 1100, andalso the regions which will correspond to the bond line 1400 between theplate substrate 1000 and the via substrate 2000 of the dual substrateelectrostatic MEMS plate switch 100. This bond line area 1400 ofmetallization will form, along with a layer of indium, a seal which willhermetically seal the plate substrate 1000 with the via substrate 2000,as will be described further below.

While a Cr/Au multilayer is disclosed as being usable for themetallization layer of the shunt bar 1100, it should be understood thatthis multilayer is exemplary only, and that any other choice ofconductive materials or multilayers having suitable electronic transportproperties may be used in place of the Cr/Au multilayer disclosed here.For example, other materials, such as titanium (Ti) or titanium tungsten(TiW) may be used as an adhesion layer between the Si and the Au. Otherexotic materials, such as ruthenium (Ru) or palladium (Pd) can bedeposited on top of the Au to improve the switch contact properties,etc. However, the choice described above may be advantageous in that itcan also participate in the sealing of the device through the alloybond, as will be described more fully below.

FIG. 9 shows the plate substrate 1000 of the dual substrateelectrostatic MEMS plate switch 100 after the silicon device layer hasbeen patterned to form the deformable plate 1300. To form the deformableplate, the surface of the device layer 1010 of the SOI plate substrate1000 is covered with photoresist which is patterned with the design ofthe deformable plate. The deformable plate outline is then etched intothe surface of the device layer by, for example, deep reactive ionetching (DRIE). Since the underlying dielectric layer 1020 has alreadybeen etched away, there are no stiction issues arising from the use of aliquid etchant, and the deformable plate is free to move upon itsformation by DRIE. As before, since the photoresist deposition andpatterning techniques are well known, they are not further describedhere.

Preparation of the plate substrate 1000 is thereby completed. Thedescription now turns to the fabrication of the via substrate 2000, asillustrated in FIGS. 10-12.

FIG. 10 shows a first step in fabricating the via substrate 2000 of thedual substrate electrostatic MEMS plate switch 100. The via substrate2000 may be, for example, silicon, glass, or any other suitable materialconsistent with the process described below, or suitable equivalentsteps. In one exemplary embodiment, the via substrate is a 500 μm thicksilicon wafer. The via substrate 2000 may be covered with a photoresist,which is patterned in areas corresponding to the locations of vias 2110,2120, 2210, 2220, 2400 and 2450, or electrical conduits that will beformed in the via substrate 2000.

Blind trenches may first be etched in the substrate 2000, for theformation of a set of vias 2110, 2120, 2210, 2220, 2400 and 2450 whichwill be formed in the trenches by plating copper into the trenches. A“blind trench” is a hole or depression that does not penetrate throughthe thickness of the via substrate 2000, but instead ends in a dead endwall within the material. The etching process may be reactive ionetching (RIE) or deep reactive ion etching (DRIE), for example, whichmay form blind trenches, each with a dead-end wall. The etching processmay be timed to ensure that the vias 2110, 2120, 2210, 2220, 2400 and2450 extend substantially into the thickness of the via substrate. Forexample, the vias 2110, 2120, 2210, 2220, 2400 and 2450 may be etched toa depth of about 60 μm to about 150 μm deep into the via substrate 2000.When the vias are completed as described below, via 2450 may provideelectrical access to the deformable plate 1300, and provide a voltagefor one side of the parallel plate capacitor which may provide theelectrostatic force required to close the switch; via 2400 may provideelectrical access to the electrostatic plate 2300 which forms the otherside of the parallel plate capacitor; via 2110 may provide electricalaccess to one of the contact electrodes 2112 of the switch; via 2120 mayprovide electrical access to the other contact electrode 2122 of theswitch, and so forth. After etching the blind trenches 2100-2450, thevia substrate may be cleaned with a solvent to remove any polymers thatmay remain on the walls of the blind trenches after the dry etchprocedure.

After formation of the blind trenches 2100-2450 and cleaning thereof,the substrate 2000 may be allowed to oxidize thermally, to form a layerof silicon dioxide 2050, which electrically isolates one via from thenext, as shown in FIG. 1. The oxide may be about 2 μm thick, forexample. A seed layer (not shown) may then be deposited on the uppersurface and in the blind trenches. The seed layer may be, for example, athin layer of chromium followed by a thin layer of gold, the chromiumfor adhesion and the gold as a seed layer for the plating of copper intothe vias 2110-2450. The chromium/gold seed layer may be, for example,about 850 nm in thickness, with about 100 nm of chromium and about 750nm of gold, and may be deposited by, for example, ion beam deposition(IBD) to provide an electrically continuous film of plating base to thebottom and sides of the vias. Metals, such as Cu, may also be depositedusing chemical vapor deposition (CVD) methods, so long as the metal is acompatible seed layer for the conductive material to be subsequentlyplated into the blind trench.

In order to fill the blind trenches 2100-2450 completely with theconductive material, the seed layer may be plated usingreverse-pulse-plating, as described in more detail in co-pending U.S.patent application Ser. No. 11/482,944, incorporated by reference hereinin its entirety.

The blind trenches 2110-2450 may then be plated with copper, forexample, or any other suitable conductive material that can be platedinto the blind trenches, such as gold (Au) or nickel (Ni), to createvias 2110-2450. To assure a complete fill, the plating process may beperformed until the plated material fills the blind trenches to a pointup and over the surface of the substrate 2000. The surface of thesubstrate 2000 may then be planarized, using, for example, chemicalmechanical planarization, until the plated vias 2110-2450 are flush withthe surface of the substrate 2000, as shown in FIG. 10. Theplanarization process may stop on the seed layer or the dielectric layer2050 of the substrate, leaving for example, about 1 μm of the previouslygrown dielectric layer 2050, which continues to provide electricalisolation between the interior metal structures of the devices, whichwould otherwise be electrically connected by the silicon via substrate2000.

A standoff 2500 may then be formed on the substrate 2000, as shown inFIG. 10. This standoff may determine the separation between the platesubstrate 1000 bearing the deformable plate 1300 and the via substrate2000, when the two substrates are bonded together. Any mechanicallyrigid material may be used, which is capable of forming a sufficientlystiff standoff. In one convenient embodiment, a polymer such asphotoresist is patterned and cured for use as standoffs 2510 and 2520.The polymer may be, for example, about 1 μm in thickness. Thephotoresist may be deposited and patterned, after which the remainingphotoresist portions 2500 may be baked to completely cure thesestructures. The negative tone photoresist SU-8, developed by IBM ofArmonk, N.Y., may be a suitable material for forming the standoff 2500.

Another metallization layer is then deposited over the substrate 2000,as shown in FIG. 11, which will form the bond ring 2600 as well ascontact electrodes 2112, 2122, 2212 and 2222. Metallization region 2300is also deposited in this step, which will form the adjacent electrodein the parallel plate capacitor of the switch. In one exemplaryembodiment, the metallization layer may actually be a multilayer ofCr/Au, the same multilayer as was used for the metallization layer 1400on the plate substrate 1000 of the dual substrate electrostatic MEMSplate switch 100. The metallization multilayer may have similarthicknesses and may be deposited using a similar process as that used todeposit metallization layer 1400 on plate substrate 1000. Themetallization layer may also serve as a seed layer for the deposition ofindium, as described below.

Although the metallization layer is described as consisting of a thinadhesion layer of Cr, and an optional antidiffusion layer of Mo,followed by a relatively thick layer of Au, it should be understood thatthis embodiment is exemplary only, and that any material havingacceptable electrical transport characteristics may be used asmetallization layer 2600. In particular, additional exotic materials maybe deposited over the gold, to achieve particular contact properties,such as low contact resistance and improved wear.

Photoresist may then be deposited on metallization layer, and patternedto provide features needed to form contacts 2112, 2122, 2212, 2222, 2300and 2600. The photoresist is exposed and developed to correspond toregions 2100-2300 and 2600. The substrate with the Cr/Au conductivematerial may then be wet etched to produce the conductive features2100-2300 and 2600. A suitable wet etchant may be iodine/iodide for theAu and permanganate for the Cr. FIG. 13 shows greater detail of contacts2112, 2122, 2212, 2222, 2300 and 2400. Also shown in FIG. 13 arefeatures 2330, which serve as regions in which the gold electrode can beelectrically isolated from the gold which comes into contact with thedielectric standoffs 1230 when the switch is closed.

Photoresist may then again be deposited over metallization layer 2600,and patterned to provide features for the plating of an indium layer2700, as shown in FIG. 12. The indium layer 2700 will, along with the Aulayer, form a hermetic seal that will bond the plate substrate 1000 tothe via substrate 2000 of dual substrate electrostatic MEMS plate switch100. The substrate 2000 with the patterned photoresist layer may then beimmersed in an indium plating bath, such that indium layers 2700 areplated in the feature, as shown in FIG. 12. The thickness of the platedindium layer may be, for example, about 3 μm to about 6 μm, and morepreferably about 4 μm. It may be important to control the relativethickness (and therefore volume) of the indium compared to the thicknessof the Au in metallization layer 2600, such that the ratio of materialsmay be appropriate to form an alloy of stoichiometry AuIn_(x), where xis about 2. Since the molar volume of indium is about 50% greater thangold, a combined gold thickness of both wafers of about 800 nm to about1600 nm may be approximately correct to form the AuIn₂ alloy. It mayalso be important to provide sufficient gold thickness that a thin layerof gold remains on the surface of the substrate 2100 to provide goodadhesion to the substrate, after the formation of the gold/indium alloy.This can additionally be ensured by plating the indium layer narrowerthan the gold metallization layers, as shown in FIG. 12, such that thefinal volumes and ratio of gold/indium provides for a slight excess ofgold at the substrate interface.

It may be important for gold metallization 2600 be wider in extent thanthe plated indium layer 2700. The excess area may allow the indium toflow outward somewhat upon melting, without escaping the bond region,while simultaneously providing for the necessary Au/In ratios citedabove.

The two portions, the plate substrate 1000 and the via substrate 2000are now ready to be assembled to form the dual substrate electrostaticMEMS plate switch 100. The two portions may be first aligned, such thatthe metallization layers 1400 of plate substrate 1000 are registeredwith the metallization layers 2700 of the via substrate 2000. Thisplaces the plated indium layer 2700 between gold metallization layers1400 and 2600.

Methods and techniques for forming the alloy seal are further describedin U.S. patent application Ser. Nos. 11/211,625 and 11/211,622, each ofwhich is incorporated by reference herein in its entirety.

For MEMS switches that benefit from a defined ambient environment, thetwo portions 1000 and 2000 of the electrostatic MEMS plate switch 100may first be placed in a chamber which is evacuated and then filled withthe desired gas. For example, for MEMS switches to be used in telephoneapplications using relatively high voltage signals, the desired gas maybe an insulating gas such as sulfur hexafluoride (SF₆), CO₂ or a freonsuch as CCl₂F₂ or C₂Cl₂F₄. The insulating gas may then be sealed withinthe dual substrate electrostatic MEMS plate switch 100 by sealing theplate substrate 1000 with the via substrate 2000 with the alloy bondformed by layers 1400, 2600 and 2700. Alternatively, an evacuated orsub-ambient or super-ambient environment may be sealed in theelectrostatic MEMS plate switch 100 with a substantially hermetic seal.The term “substantially hermetic” may be understood to mean that theenvironment sealed with the device at manufacture retains at least about90% of its original composition over the lifetime of the device. For adevice sealed with a sub-ambient or super-ambient environment, thepressure at its end-of-life may be within about 10% of its pressure atmanufacture.

To form the alloy bond between layers 1400, 2600 and 2700, platesubstrate 1000 may be applied to the via substrate 2000 under pressureand at elevated temperature. For example, the pressure applied betweenthe plate substrate 1000 and the via substrate 2000 may be from 0.5 to2.0 atmospheres, and at an elevated temperature of about 180 degreescentigrade. This temperature exceeds the melting point of the indium(157 degrees centigrade), such that the indium flows into and forms analloy with the gold. As mentioned above, the stoichiometry of the alloymay be about 2 indium atoms per one gold atom, to form AuIn_(x) where xis about 2. In contrast to the low melting point of the indium metal,the melting point of the alloy is 541 degrees centigrade. Therefore,although the alloy is formed at a relatively low temperature, thedurability of the alloy bond is outstanding even at several hundreddegrees centigrade. The bond is therefore compatible with processeswhich deposit vulnerable materials, such as metals, on the surfaces andin the devices. These vulnerable materials may not be able to survivetemperatures in excess of about 200 degrees centigrade, withoutvolatilizing or evaporating.

Upon exceeding the melting point of the indium, the indium layers 2700flows outward, and the plate substrate 1000 and the via substrate 2000are pushed together, until their approach is stopped by the polymerstandoff 2500. As the alloy forms, it may immediately solidify, sealingthe preferred environment in the dual substrate electrostatic MEMS plateswitch 100.

While the systems and methods described here use a gold/indium alloy toseal the MEMS plate switch, it should be understood that the dualsubstrate electrostatic MEMS plate switch 100 may use any of a number ofalternative sealing methodologies, including different constituentmetals for the bond line and cross-linked polymers. For example, theseal may also be formed using a low-outgassing epoxy which isimpermeable to the insulating gas.

In order to apply the appropriate signals to contact pads 2112, 2122,2212, 2222, 2400 and 2450, electrical access may need to be achieved tovias 2112, 2122, 2212, 2222, 2400 and 2450. As described earlier, vias2110, 2120, 2210, 2220, 2400 and 2450 may begin as blind trenches formedin one side of the substrate, and plated with a conducting material. Toprovide access to the conducting vias formed in the front side, materialfrom the opposite, back side of substrate 2000 may be removed until thedead-end walls of the blind trenches 2110-2450 have been removed, suchthat electrical access to the vias may be made from the back side. Inone exemplary embodiment, the original 500 μm thick silicon wafer isbackground until it has a thickness of about 80 μm, and the vias 2110,2120, 2210, 2220, 2400 and 2450 extend through the entire thickness ofthe remaining silicon. The technique for removing the excess materialmay be, for example, grinding. The processes used to form the vias isdescribed in more detail in U.S. patent application Ser. Nos. 11/211,624and 11/482,944, incorporated by reference herein in their entireties.

The via substrate 2000 may then be coated with an oxide 2200, which maybe SiO₂, for example, at a thickness sufficient to isolate the vias2110-2220 one from the other. The oxide may be deposited by a lowtemperature dielectric deposition process, such as sputtering or plasmaenhanced chemical vapor deposition (PECVD) to a thickness of about 1 μm.The oxide-coated substrate 2000 may then be covered with photoresist andpatterned to form openings at the locations of the vias 2110-2145. Thesubstrate 2000 may then be etched through the photoresist to remove theoxide 2200 from the backside openings of the vias 2110-2450. Thephotoresist may then be stripped from the substrate 2000. Since theseprocesses are well known in the art, they are not described or depictedfurther.

The rear surface of substrate 2100 may then be covered with a conductivelayer. In some exemplary embodiments, the conductive layer may be aCr/Au multilayer, chosen for the same reasons as multilayers 1900 and2600, and deposited using the same or similar techniques. Alternatively,the conductive layer may be any conductive material having acceptableelectrical and/or thermal transport characteristics. In one exemplaryembodiment, the conductive material may be a multilayer of 15 nmchromium, followed by 800 nm of nickel, and finally 150 nm of gold. Thenickel may give the multilayer better wear and durabilitycharacteristics than the gold alone over the chromium layer, which maybe important as these features are formed on the exterior of theelectrostatic MEMS plate switch 100.

The conductive layer is then covered once more with photoresist, whichis also patterned with features which correspond to pads 2115, 2125,2405 and 2455 on the backside of the dual substrate electrostatic MEMSplate switch 100. Alternatively, the metal may be deposited through ashadow mask, allowing for the possibility of thicker layers andeliminating the need for further processing.

The conductive layer on the rear of the substrate 2000 is then etched orion milled, for example, to remove the conductive layer at the openingsof the photoresist, to form isolated conductive bonding pads 2115, 2125,2405 and 2455. Conductive bonding pad 2115 may provide electrical accessto the contact points 2110 and 2120 of the switch; conductive bondingpad 2125 may provide electrical access to the contact points 2210 and2220 of the switch; conductive bonding pad 2405 may provide electricalaccess to via 2400 and adjacent electrode 2300 of the switch; andconductive bonding pad 2455 may provide the ground signal to the dualsubstrate MEMS electrostatic MEMS plate switch 100. These bonding pads2115, 2125, 2405 and 2455 are shown in the plan view of the back side ofthe via substrate in FIG. 14. After formation of bonding pads 2115,2125, 2405 and 2455, the electrostatic MEMS plate switch is essentiallycomplete, and the wafer pair 1000 and 2000 may be sawed and/or diced toseparate the individual electrostatic MEMS plates switches from theadjacent devices formed on the wafers.

FIG. 15 shows an individual dual substrate electrostatic MEMS plateswitch 100 after manufacture and assembly. In its completed state, theshunt bar 1100 on the deformable plate 1300 hangs adjacent to andspanning the electrical contacts 2110 and 2120, and the deformable plate1300 is also adjacent to the metallization plate electrode 2300, asshown in FIG. 1. Upon applying appropriate voltages to vias 2400 and2450 using conductive bonding pads 2405 and 2455, respectively, adifferential voltage forms across the parallel plate capacitor formed bythe deformable plate 1300 and the electrode 2300, drawing the deformableplate 1300 toward the electrode 2300. At its lower point of travel orvibration of the deformable plate 1300, the shunt bar 1110 affixed tothe deformable plate 1300 is applied across the electrical contacts 2110and 2120 of the dual substrate electrostatic MEMS plate switch 100, andshunt bar 1120 is applied across electrical contacts 2210 and 2220,thereby closing the switch. An input electrical signal applied to one ofthe electrical contacts 2110 and 2210 by conductive bonding pad 2115 maythen be obtained as an output electrical signal from either of the othercontacts 2120 or 2220 by the other conductive bonding pad 2125. Theswitch may be opened by discontinuing the voltages applied to the plate1300 and electrode 2300, whereupon the switch may return to its originalposition because of the restoring spring force acting on the stiffspring beams 1330 coupled to the deformable plate 1300.

Exemplary thicknesses of various layers of the dual substrateelectrostatic MEMS plate switch 100 are shown in FIG. 16. It should beunderstood that the features depicted in FIG. 16 may not necessarily bedrawn to scale. As shown in FIG. 16, an exemplary thickness of the Cr/Auconductive layer 2600 is about 0.75 μm. An exemplary distance h betweenthe upper surface of the shunt bar 1100 and the lower surface of thecontact point 2112, also defined as the throw of the switch, may be, forexample, about 1.0 μm. An exemplary thickness of the conductive materialof the shunt bar 1100 and contacts 2122 and 2112 may be, for example,about 0.75 μm each. An exemplary thickness of the deformable plate 1300may be about 5.0 μm, which may also be the thickness of the device layer1010. An exemplary thickness of the isolation layer 1200 may be about0.3 μm. Finally, an exemplary thickness t1 of the polymer standoff 2520may be about 1.0 μm, which also defines a minimum separation between theplate substrate 1000 and the via substrate 2000, of the dual substrateelectrostatic MEMS plate switch 100. An exemplary thickness t2 of thealloy bond (In material as well as Cr/Au multilayers) may be about 1.7μm. It should be understood that the dimensions set forth here areexemplary only, and that other dimensions may be chosen depending on therequirements of the application.

A single contact plate switch 300 may also be fabricated using a processvery similar to the one described above for the dual substrateelectrostatic MEMS plate switch 100. The single contact plate switch 300may have only a single junction or contact between the input line andthe output line, compared to dual substrate electrostatic MEMS plateswitch 100, which may have at least two junctions spanned by a movableshunt bar. The single junction or contact can be opened or closed byactivating the switch. The single contact plate switch 300 may have theadvantage of lower contact resistance, because there is only a singlejunction between the input line and the output line. In addition, thesingle contact plate switch 300 may have superior current handlingcharacteristics, because heat built up in the shunt bar may beefficiently dissipated into the via substrate, as there is no highresistance junction impeding this heat flow out of the shunt bar.Furthermore, since there are no longer two contacts, the deformableplate need not flex or gimbal to accommodate any mismatch between theelevations of the contacts. Finally, since only one contact needs to beclosed rather than two, the electrostatic force needed to close theswitch may be reduced by a factor of two. This may allow the singlecontact plate switch 300 to be reduced in size compared to electrostaticMEMS plate switch 100. Accordingly, the single contact plate switch 300may have a number of improved performance attributes relative to theelectrostatic MEMS plate switch 100, while retaining all themanufacturing advantages of the electrostatic MEMS plate switch outlinedabove.

Like dual substrate MEMS plate switch 100, the single contact plateswitch 300 may also include a deformable plate 3300 which is shown inFIG. 17. The deformable plate 3300 may also be suspended adjacent to aplate substrate 3000 by four spring beams 3330. The deformable plate3300 may be formed in the device layer of an SOI substrate, similar tothe construction of electrostatic MEMS plate switch 100. If the platesubstrate 3000 is an SOI substrate, the spring beams 3330 may secure thedeformable plate 3300 to the plate substrate by attachment points whichare areas in the SOI substrate in which the dielectric layer underlyingthe device layer has not been removed.

FIG. 17 shows an embodiment of the single contact MEMS plate switch withthe four spring beams 3330 arranged symmetrically around the deformableplate 3300. This embodiment is similar to the symmetric embodiment ofdual substrate MEMS plate switch shown in FIG. 3. Although a symmetricembodiment is shown in FIG. 17, it should be understood that the singlecontact plate switch 300 may also be designed with an asymmetricorientation of the spring beams, in which the beam springs disposed onone side of the deformable plate are anti-symmetric with respect to thebeams springs disposed on the opposite side of the deformable plate.Thus, when the asymmetric deformable plate with spring beams isreflected across a longitudinal or latitudinal axis, the spring beamsextend in an opposite direction from the bend. Thus, this asymmetricembodiment is similar to the asymmetric embodiment of dual substrateMEMS plate switch illustrated in FIG. 5. Like asymmetric dual substrateMEMS plate switch illustrated in FIG. 5, the asymmetric single contactMEMS plate switch may have additional rotational motion upon closing,which may allow the contact surfaces to scrub against each other,providing a lower contact resistance.

The deformable plate 3300 of the single contact MEMS plate switch mayhave etch release holes 3310, similar in design and function to etchrelease holes 1310 in electrostatic MEMS plate switch 100, and may bemade using similar processes to those described above with respect toetch release holes 1310.

The plate 3300 may also have a plurality of dielectric standoffs 3230,which are again of similar form and function to dielectric standoffs1200, 1210, 1220 and 1230 of electrostatic MEMS plate switch 100, andmay be made using processes described above with respect to dielectricstandoffs 1200, 1210, 1220, and 1230. These dielectric standoffs mayprevent the plate 3300 from shorting to an adjacent electrode when theswitch is closed.

An important difference between the deformable plate 3300 and deformableplate 1300 of electrostatic MEMS plate switch 100 is with respect to thedesign of the shunt bar 3100. As with deformable plate 1300, the shuntbar 3100 may be disposed over a dielectric layer 3200 which may isolatesignals traveling in the shunt bar 3100 from the deformable plate 3300.However, in the case of the single contact plate switch 300, the shuntbar 3100 may be attached mechanically to the deformable plate 3300 onlyat one end 3110. The other end 3120 of the shunt bar 3100 may beattached to a second electrode (not shown in FIG. 17) located on anothermember. These attachment points are herein referred to as the moving endor moving contact 3110 and the stationary end or stationary contact3120, because the moving contact 3110 is affixed to, and moves with, thedeformable plate 3300, whereas the stationary contact 3120 is fixed.These features are shown more clearly in the perspective view of FIG.18.

In the exemplary embodiment described below, the stationary end 3120 isaffixed to a second electrode formed in a second, via substrate 4000disposed adjacent to the plate substrate 3000. This embodiment isanalogous to dual substrate MEMS switch 100, wherein the deformableplate 1300 is formed in a plate substrate 1000, and the vias are formedin an adjacent via substrate 2000. The two substrates are then mated toform the single contact MEMS plate switch 300.

Accordingly, when the switch is open, the shunt bar 3100 spans its twoends which are secured to different members: moving end 3110 which issecured to the moving, deformable plate 3300, and stationary end 3120which is secured to the second electrode 4122 on the adjacent viasubstrate 4000. In the open, quiescent state, a gap exists between themoving contact 3110 and the first electrode 4112 formed in the viasubstrate. When the switch 300 closes, the electrostatic force pulls thedeformable plate 3300 toward an adjacent electrostatic plate located onthe second substrate to close the gap. The deformable plate therebypushes the moving contact 3110 against the first electrode 4112 formedin the via substrate 4000 to close the switch, and allow a signal toflow from the first electrode 4112 in the via substrate to the secondelectrode 4122 in the via substrate.

FIG. 18 is a perspective view of deformable plate 3300 and the adjacentelectrodes 4112 and 4122. These electrodes 4112 and 4122 are locatedabove through wafer vias 4110 and 4120, respectively, which aredescribed below with respect to FIGS. 20 and 24-26. As shown in FIG. 18,when the deformable plate 3300 is in its as-manufactured, open positionbefore activation of the electrodes, the moving end 3110 is suspendedabove its respective contact point 4112 and via 4110. In the openposition, the shunt bar may be approximately parallel to its fabricationsubstrate. In its closed position, however, the shunt bar 3100 isrequired to flex between its attachment point to the second electrodeand its contact position with the first electrode.

To allow this flexing of the shunt bar 3100 out of the plane of thedeformable plate 3300, the material of the deformable plate 3300 may beremoved in areas 3340 near the contacts 3110 and 3120, where the shuntbar 3300 needs to flex. This cut-out area, or void 3340 is shown in FIG.18. In some embodiments, the void 3340 may extend laterally somedistance from the first contact 3120 in a direction toward the secondcontact 4112, as shown in FIG. 18.

In other embodiments, void 3340 may actually include two voids, a firstformed around the stationary end 3120 and a second formed near themoving end 3110, wherein the first void is separated from the secondvoid by a narrow isthmus of material. The isthmus of material may remainat least partially under the moving contact 3110, in order to urge themoving contact 3110 against the first electrode via 4110 formed in thevia substrate 4000, in the deflected, closed position. Such embodimentsare described in further detail with respect to FIGS. 27-29, below.

A three-dimensional perspective view of the deformable plate 3300 andshunt bar 3100 of single contact plate switch 300 is shown in itsdeflected, closed position in FIG. 19, i.e., after actuation of thedeformable plate 3300. The deformable plate 3300 is shown from the rear,i.e. the contact points 4110 and 4120 lie beneath and are obscured bydeformable plate 3300. In this position, the shunt bar 3100 flexesbetween its stationary attachment point 3120 coupled to the viasubstrate (not shown) and its moving attachment point 3110 coupled tothe deformable plate 3300. The single contact MEMS plate switch 300 isshown in the same relative orientation in FIG. 19 as in FIGS. 17 and 18.When the switch is activated, electrostatic forces draw the plate 3300down until the moving contact 3110 attached to the deformable plate 3300touches the first electrode contact 4110 adjacent to it, and located onthe via substrate. Accordingly, both the spring beams 3330 as well asthe flexed shunt bar 3100 provide the restoring force which returns thedeformable plate 3300 to its original position when the switch isdeactivated, and the switch is opened.

The single contact plate switch 300 is shown in its entirety, includingboth the plate substrate 3000 and the via substrate 4000 in crosssection in FIG. 20. In FIG. 20, like numbers correspond to analogousfeatures shown in FIGS. 15 and 16 for the electrostatic MEMS plateswitch 100, and can be made using processes described above with respectto the features shown in FIGS. 15 and 16. For example, reference number3300 designates the deformable plate, which corresponds with referencenumber 1300 of electrostatic MEMS plate switch 100. FIG. 20 shows thetwo vias 4120 and 4110 located adjacent to the fixed end 3120 and movingend 3110 of the shunt bar 3100, respectively. The vias 4110 and 4120 maybe covered by a contact layer of gold 4112 and 4122, respectively, toform the junction layer over the vias which makes contact to the shuntbar 3100. These features may be analogous to vias and contacts 2110,2112, 2120 and 2122 shown in FIG. 16.

This same layer of gold as used for contact layers 4112 and 4122 mayform a part of the AuIn hermetic bond as in the electrostatic MEMS plateswitch 100, and may be formed as described above with respect to theelectrostatic MEMS plate switch 100. The vias 4110 and 4120 and contactlayers 4112 and 4122 may be formed using similar methods to those usedto form the corresponding features 2110 and 2120 and layers 2112 and2122 of the electrostatic MEMS plate switch 100, described above.

The standoffs 3500 in the single contact MEMS plate switch 300embodiment, however, may be formed on the plate substrate 3000 ratherthan the via substrate 4000. It should be understood that this isexemplary only, and that the standoff 3500 may be formed on eithersubstrate. These standoffs 3500 may then participate in the bonding ofthe stationary end 3120 of the shunt bar 3100 to the via substrate 4000,as well as forming the hermetic seal around the device. Otherwise,processes for forming the insulating layer 4050, vias 4120 and 4110 maybe formed as described above with respect to features 2050, 2120 and2110 of electrostatic plate switch 100, with the thicknesses asdescribed above for these features.

The process for fabricating the plate substrate 3000 is also similar tothat described above with respect to plate substrate 1000 except forformation of the shunt bar 3100. As described above, the shunt bar 3100needs to be suspended in areas above the plate substrate 3000, and havedimensions which allow it adequate flexibility to open and close theswitch 300. Exemplary dimensions for a shunt bar 3100 and deformableplate 3300 are given below for a switch which activates within a voltagerange of about 35-50 V. An exemplary process for forming the suspendedshunt bar 3100 is illustrated in FIGS. 19-21, and will be describednext.

The suspended shunt bar 3100 may be formed by first inlaying asacrificial material into the device layer 3300 of the SOI platesubstrate 3000. The later removal of this inlaid material may form partof the void required to allow the shunt bar 3100 to flex out of theplane of the deformable plate. The sacrificial material may be anymaterial which may be easily removed later in the processing by asuitable etchant, for example, Ni, Ni alloys or Cu. In one exemplaryembodiment, the sacrificial layer may be plated nickel-iron (NiFe) whichis plated into a hole 3350 left by deep reactive ion etching (DRIE) afeature 3350 in the device layer 3300. The hole 3350 may be formed byapplying photoresist to the plate substrate 3000, patterning the resist,and performing DRIE to the exposed areas. The DRIE may proceed until thedepth of the hole reaches the underlying insulating layer of the SOIsubstrate. FIG. 21 is a cross sectional view of the plate substrate 3000after formation of the hole 3350. The buried SiO₂ layer may also beetched in a subsequent reactive ion etch (RIE) process, whichpreferentially etches SiO₂ over Si using the same mask pattern, creatinga total cavity depth equal to the SOI silicon device layer thicknessplus the buried oxide thickness.

The sacrificial material 3360 may then be deposited into the hole by,for example, plating onto a seed layer also deposited in the hole. Theplated sacrificial material may then be planarized using, for example,chemical mechanical planarization (CMP). A CMP etch stop, using a veryhard material, such as silicon nitride (Si₃N₄), titanium tungstennitride (TiWN) or tantalum nitride (TaN) may be deposited over the wafereither before the initial cavity etch, or just prior to the seed layerdeposition, to protect the SOI Si surface during the CMP process. Theetch stop may be deposited using LPCVD, PECVD or PVD techniques, andthen removed post CMP using reactive ion etching or wet etching.Additional details of the plating and planarizing techniques for thesacrificial layer may be found in U.S. patent application Ser. No.11/705,739, incorporated by reference in its entirety. The seed layermay be chromium (Cr) and/or nickel (Ni), deposited by chemical vapordeposition (CVD) or sputter deposition to a thickness of 100-200 nm.Photoresist may then be deposited over the seed layer, and patterned byexposure through a mask corresponding to the desired width and length ofthe sacrificial material 3360. The sacrificial material 3360 may then beplated into the trenches formed in the patterned photoresist. Suchtechniques are well know in the MEMS art, and thus additional detailsare not provided here. For an SOI wafer with a 5 μm device Si layer and2 μm buried SiO₂ layer, the dimensions of the inlayed, platedsacrificial material may be 40 μm wide by 80 μm long by 7 μm thick. Thesacrificial thickness may be equal to the SOI device layer thicknessplus the buried SiO₂ layer thickness, as stated above. Another importantaspect of the sacrificial layer is that it may completely surroundislands of Si that may later form the fixed contact of the device. Thisisland of silicon and underlying buried silicon dioxide may not beetched by the hydrofluoric acid (HF) processes that follow, thus thesacrificial material may also effectively function as an HF barrier.

After the deposition and planarization of the sacrificial material 3360,oxide features 3200 and 3230 may be formed on the plate substrate 3000.The methods for forming the oxide features 3200 and 3230 may be similarto those used to form oxide barriers 1230 for the dual substrate MEMSplate switch 100, described above. Oxide feature 3200 may serve toisolate the conductive shunt bar 3100 which will be deposited over oxidefeature 3200 from the deformable plate 3300. Oxide features 3230 mayprevent the deformable plate 3300 from shorting to the via substrate4000 when the switch is activated. The oxide features are shown in thecross section of FIG. 22.

A standoff material 3500 such as photoresist may then be formed in areaswhere it is needed for bonding. These areas include the bondline aroundthe device and the region beneath what will be the stationary contact ofthe shunt bar 3200. This standoff is analogous to standoff 2500 shown inFIG. 16, and may be formed using similar processes. The standoff servesthe same purpose as standoff 2500, in aiding the formation of a robusthermetic seal and also control the wafer-to-wafer spacing precisely.

The shunt bar conductive material 3100 may then be deposited over theoxide 3200, the standoff 3500 the sacrificial material 3360. Amultilayer of 20 nm sputtered TiW/100 nm sputtered Au/1 μm plated AuPdmay serve as the conductive material of the shunt bar 3100. Thismultilayer may also serve as the contact surfaces 4112 and 4212 of thethrough wafer vias 4110 and 4120, as well as participating in the bondline 3600 to form the hermetic seal around the device. It should beunderstood that these materials and thicknesses are exemplary only, andthat any of a variety of other conductive materials and otherthicknesses may be used for these features. Furthermore, other bondingmaterials, such as epoxy, cement, glue, or glass frit may be used inplace of the metal alloy bond 3600. FIG. 23 shows a cross sectional viewof the shunt bar conductive material 3100, bondline 3600 and underlyingstandoffs 3500.The shunt may also be made with plurality ofperforations, which can serve to minimize the release time and alsomodify the stiffness of the shunt in multiple deformation modes.

After formation of the shunt bar, the sacrificial material under theshunt bar may be selectively etched out from under the shunt bar, usingcommercially available liquid etchants, such as ferric chloride for Niand Ni alloys, and sulfuric acid and hydrogen peroxide for Cu. Inanother variation of the process, the sacrificial material etch may bedone after the beam is etched in the next step below.

As a last step in the formation of the plate substrate 3000, thedeformable plate 3300 may be formed by deep reactive ion etching throughthe thickness of the device layer 3010 of the SOI substrate, along theoutline of the deformable plate 3300. The cut-out area 3340 may beformed simultaneously with the formation of the deformable plate 3300.These processes are similar or identical to those described above withrespect to the formation of deformable plate 1300 of dual substrate MEMSplate switch 100. The condition of the plate substrate after formationof the deformable plate is shown in cross section in FIG. 24.

FIG. 24 is a cross sectional view of the via substrate 4000 registeredabove the plate substrate 3000, before bonding the pair of substratestogether. The via substrate 4000 may be fabricated with at least twothrough wafer vias 4110 and 4120 which correspond to the fixed end 3110and moving end 3120 of the shunt bar 3100. At least one additional via4310 may provide a voltage to electrostatic plate 4300, which willinteract with plate 3300 to close the single contact electrostatic MEMSplate switch 300. The through wafer vias 4110 and 4120 may be coveredwith conductive contacts 4112 and 4122, respectively, which may beformed using similar methods to those set forth above with respect tovias 2210, 2220 and 2400 of electrostatic MEMS plate switch 100. In oneembodiment, the conductive contacts may be multilayers of 20 nmsputtered TiW/100 nm sputtered Au/1 μm plated AuPd, which is the samemultilayer used for the shunt bar 3100. In areas where the 20 nmsputtered TiW/100 nm sputtered Au/1 μm plated AuPd multilayer will formpart of the bondline, this multilayer may be covered with a 2 μm thicklayer of indium. As described above with respect to indium layer 2700,the indium layer may be deposited by plating. Upon heating the device,the indium will melt into the gold of the multilayer to form a hermeticalloy seal, as described more fully above with respect to electrostaticMEMS plate switch 100.

To complete fabrication of the single contact electrostatic MEMS plateswitch 300, the via substrate 4000 may be brought adjacent to the platesubstrate 3000 and aligned as described above with respect to thefeatures of the plate substrate 3000. The two wafers may be held inplace using, for example, a clamp and the pair may then be inserted intoa wafer bonding chamber. The wafer bonding chamber may be filled with apreferred gas environment, in order to seal this environment in thedevice. The via substrate 4000 may then be pressed against the platesubstrate 3000 in the wafer bonding chamber, and heated to melt theindium metal. As the AuIn alloy forms, it immediately solidifies to formthe hermetic seal. The completed device is shown in FIG. 25.

The single contact MEMS plate switch 300 may also be formed on asubstrate prepared with voids 3700 under the device layer 3010, prior toprocessing of the device layer 3010. In this embodiment, the voids 3700may be formed by performing an etching process on the handle layer 3030and dielectric layer 3020 of an SOI substrate by, for example, deepreactive ion etching. The device layer 3010 may then be deposited orbonded to the remaining substrate. Such an embodiment is illustrated inFIG. 26, wherein voids 3700 are shown formed in the handle layer 3030and dielectric layer 3020 of the SOI substrate 3000. Such a substrate issometimes called an “engineered” substrate, as features may be formed inthe substrate which are advantageous to the later processing orfunctioning of the device. The device layer 3030 may then be processedas described above to form single contact MEMS switch 300. The voids3700 may provide larger reservoirs of air to reduce squeeze film dampingand thereby improve the switching speed of single contact MEMS switch300. Process benefits may be realized through the use of an engineeredsubstrate. In the previously described process, holes must be formed atthe beginning of the process to provide access for etchant to removeburied SOI oxide and form a cavity under what will later become thebeam. The use of an engineered substrate eliminates these holes, and thepossibility of getting photoresist, process chemicals and sputteredmetal under the beam during processing.

FIG. 27 is a simplified diagram of additional embodiments of the singlecontact MEMS plate switch 300. FIG. 27 is intended to illustrate thebasic shape and functioning of the cut-out, void portion 3340surrounding the stationary contact 3120 of the shunt bar 3100. In thefirst exemplary embodiment, the cut-out, void portion 3340 may be twosimple rectangular shapes, joined by a thin isthmus of material 3350lying across a region of the moving contact 3110. The isthmus ofmaterial 3350 may be an integral part of the deformable plate 3300, andmay be rigidly coupled to the moving contact 3110.

The isthmus of material 3350 functions to close the switch when theelectrostatic plate 4300 beneath the deformable plate 3300 is activatedwith a voltage that pulls the deformable plate 3300 toward the oppositeelectrode. When the electrostatic plate 4300 is activated, deformableplate 3300 is drawn toward the electrostatic plate 4300, along with theisthmus of material 3350. Since the isthmus of material 3350 is attachedto the moving contact 3110, the moving contact 3110 is pushed againstthe second contact 4112 formed in the via substrate 4000, to close theswitch. The shape of the isthmus of material 3350 may determine thetrajectory with which the moving contact 3110 is lowered onto theadjacent contact 4112 in the via substrate, 4000.

FIG. 28 is a simplified illustration of another exemplary embodiment ofthe cut-out, void portion 3340 of the deformable plate 3300. In thisexemplary embodiment, the cut-out, void portion 3340′ has an extendedisthmus of material 3350′, which may extend from a ninety-degree bendadjacent to the contact area between the moving contact 3110 and thesecond contact 4112. As the deformable plate is lowered, this longeristhmus provides some amount of lateral movement of the moving contact3110 over the second via contact 4112, as indicated by the doublearrowhead in FIG. 28. This lateral movement may produce some “scrubbing”of the contact surfaces as the contact is made. This movement may resultin a lower contact resistance across the contacts and lower thepropensity of a failure due to welding, as the scrubbing may loosen anycontamination or debris that may have accumulated on the contactsurfaces and can also apply a torque to break microscopic welds.

FIG. 29 is a simplified illustration of yet another embodiment of thecut-out, void portion 3340″ of the deformable plate 3300. In thisembodiment, the isthmus of material 3350″ is formed with two bends toform an “S”-like shape in the isthmus of material 3350″. Thisarrangement gives the isthmus of material 3350″ some rotationalflexibility as well as lateral flexibility similar to the precedingembodiment. Thus, isthmus of material 3350″ may rotate as well astranslate laterally when being lowered onto the second adjacent contact4112. An arrow indicates a direction of rotation in FIG. 29. Thisembodiment may have advantages in situations where more scrubbing orrotational movement is useful.

The arrangement of the spring beams 3330 in these embodiments may beeither in the symmetrical arrangement, as shown in FIG. 28, or in theasymmetrical arrangement, as shown in FIG. 29. The symmetricalarrangement is analogous to the symmetrical arrangement of MEMS plateswitch 100 shown in FIG. 3, and the asymmetrical arrangement isanalogous to the asymmetrical arrangement of MEMS plate switch 100 shownin FIG. 5. It should be understood that the cut-out, void portion 3340′may be used with either the symmetrical or asymmetrical arrangement ofthe spring beams, and the cut-out, void portion 3340″ may likewise beused with either the symmetrical or asymmetrical arrangement of springbeams. FIGS. 28 and 29 illustrate only one particular combination ofthese elements, and it should be understood that other combinations arealso anticipated.

What follows is a description of one particular embodiment of a designfor a single contact MEMS plate switch 300. It should be understood thatthe dimensions set forth below are exemplary only, and may be adjustedfor the requirements of a particular application. The deformable plate3300 may be, for example, about 200 μm on a side, and the same thicknessas the device layer from which it is made, about 5 μm thick. Thecontacts 4110 and 4120 may be copper plated vias in the via substrate4000, about 15 μm in diameter and 45 μm to 90 μm deep, through thethickness of the via substrate 4000. The Cu vias may be topped withlayers 4112 and 4122, which may be multilayers of metals and metalalloys, such as Ni, W, Au, and AuPd, with a thickness of about 1 μm anda diameter of about 20 μm. These layers may be deposited as describedabove, using for example, sputtering or electroplating.

The spring beams 3330 may be formed by deep reactive ion etching throughthe device layer of an SOI substrate, and may be about 4 to 10 μm wide,134 μm long and about 5 μm thick. The four spring beams together maygenerate a restoring force of about 100 μN. The shunt bar 3100 may be,for example, about 70 μm long, 20 μm wide, and 0.5 μm thick, and made ofplated or ion beam deposited gold, for example. With these dimensions,the shunt bar may have a stiffness of about 6.4 N/m, so may offer aforce of about 10 μN against the pull down force of about 400 μN. Inthis exemplary design, the single contact MEMS plate switch 300 isintended to operate at a switching voltage in the range of about 35-50V.

The cut-out area 3340 in the deformable plate 3300 may be, for example,100 μm long and about 40 μm wide. The cut-out area 3340 may be formed bydeep reactive ion etching, and may be formed during formation of thedeformable plate 3300 itself. Within the cut-out area 3340, the isthmusof material 3350 may be about 4 μm wide, 40 μm long and about 5 μmthick, and also formed by deep reactive ion etching.

While various details have been described in conjunction with theexemplary implementations outlined above, various alternatives,modifications, variations, improvements, and/or substantial equivalents,whether known or that are or may be presently unforeseen, may becomeapparent upon reviewing the foregoing disclosure. For example, while thedisclosure describes a number of fabrication steps and exemplarythicknesses for the layers included in the MEMS switch, it should beunderstood that these details are exemplary only, and that the systemsand methods disclosed here may be applied to any number of alternativeMEMS or non-MEMS devices. Furthermore, although the embodiment describedherein pertains primarily to an electrical switch, it should beunderstood that various other devices may be used with the systems andmethods described herein, including actuators and valves, for example.Accordingly, the exemplary implementations set forth above, are intendedto be illustrative, not limiting.

What is claimed is:
 1. A method for manufacturing a switch, comprising:forming a first plate suspended adjacent to a first substrate by atleast one spring beam; coupling at least one conductive bar to the firstplate at a location on the at least one conductive bar; coupling the atleast one conductive bar to a first contact, wherein the first contactcan protrude into without touching the first plate, at another locationon the at least one conductive bar; and configuring the first plate toactivate the switch by pressing the at least one conductive bar againsta second contact.
 2. The method of claim 1, wherein the first contact isformed on a second substrate.
 3. The method of claim 2, furthercomprising: forming an electrostatic second plate and the second contacton the second substrate; aligning the first substrate to the secondsubstrate; and coupling the first substrate to the second substrate witha seal.
 4. The method of claim 1, wherein the conductive bar iselectrically isolated from the first plate by a dielectric layer.
 5. Themethod of claim 3, further comprising: forming at least one electricalvia through a thickness of the second substrate, and electricallycoupling the at least one electrical via to the second contact.
 6. Themethod of claim 5, wherein forming the at least one electrical viacomprises: forming at least one blind hole with a dead end wall on afront side of the second substrate; forming a seed layer in the at leastone blind hole; depositing a conductive material onto the seed layer;and removing material from a rear side of the second substrate to removethe dead-end wall of the at least one blind hole.
 7. The method of claim1, wherein forming the at least one conductive bar further comprises:forming a cavity in the first substrate; filling the cavity with asacrificial material; depositing conductive material of the conductivebar over the cavity filled with the sacrificial material; and removingat least a portion of the sacrificial material beneath the conductivematerial of the conductive bar.
 8. The method of claim 7, whereinforming the at least one conductive bar further comprises: forming afirst void in the first plate that extends laterally from the firstcontact toward the second contact.
 9. The method of claim 8, furthercomprising; forming a second void in the first plate near the secondcontact, wherein the first void is separated from the second void by anarrow isthmus of material.
 10. The method of claim 9, wherein theisthmus is configured to have at least one of lateral and rotationalmovement as the switch is activated.
 11. The method of claim 3, whereincoupling the first substrate to the second substrate with a sealcomprises: depositing a first metal between the first substrate and thesecond substrate; and depositing a second metal between the firstsubstrate and the second substrate; and coupling the first substrate tothe second substrate by heating the first metal and the second metal toat least a melting point of at least one of the first metal and thesecond metal, to form a substantially hermetic alloy seal around theswitch.
 12. The method of claim 1, wherein the first substrate comprisesa silicon-on-insulator substrate, and the second substrate comprises atleast one of a silicon wafer and a silicon-on-insulator substrate. 13.The method of claim 8, wherein forming the first plate suspended overthe first substrate comprises: etching a plurality of holes into adevice layer of the silicon-on-insulator substrate; etching a dielectriclayer beneath the device layer of the silicon-on-insulator substratethrough the plurality of holes; and etching an outline of the firstplate in the device layer of the silicon-on-insulator substrate.
 14. Aswitch, comprising: a first plate suspended adjacent to a firstsubstrate and coupled to the first substrate by at least one springbeam; at least one conductive bar coupled to the first plate at alocation on the at least one conductive bar, and coupled to a firstcontact at another location on the at least one conductive bar, whereinthe first contact can protrude into without touching the first plate,wherein the first plate is configured to activate the switch by pressingthe at least one conductive bar against a second contact.
 15. The switchof claim 14, wherein the second contact is coupled to a second substrateand wherein a hermetic seal couples the first substrate to the secondsubstrate, to enclose the switch.
 16. The switch of claim 14, whereinthe at least one spring beam comprises at least two spring beams, atleast one of the two spring beams disposed on one side of the firstplate, and at least one other of the at least two spring beams disposedon an opposite side of the first plate, wherein each spring beam has asegment extending from the first plate which is coupled to an adjoiningsegment by a bend.
 17. The switch of claim 14, wherein the first plateand spring beams are substantially symmetric about at least one of alongitudinal and latitudinal axis of the first plate.
 18. The switch ofclaim 14, wherein the at least one spring beam disposed on one side ofthe first plate is substantially anti-symmetric with respect to the atleast one other spring beam disposed on an opposite side of the firstplate.
 19. The switch of claim 15, wherein the first substrate is asilicon-on-insulator substrate comprising a device layer, a handle layerand a dielectric layer between the device layer and the handle layer,and the second substrate is at least one of a silicon substrate and asilicon-on-insulator substrate.
 20. The switch of claim 15, furthercomprising: electrical vias formed through a thickness of the secondsubstrate; and an electrostatic second plate formed on the secondsubstrate.
 21. The switch of claim 14, further comprising: a first voidformed around the first contact.
 22. The switch of claim 21, furthercomprising: a second void formed near the second contact, wherein thefirst void is separated from the second void by an isthmus of material.23. The switch of claim 22, wherein the isthmus of material isconfigured to have at least one of lateral and rotational movement uponactivation of the switch.
 24. The switch of claim 15, wherein thehermetic seal comprises: a gold/indium alloy which bonds the firstsubstrate to the second substrate with a substantially hermetic sealaround the switch.
 25. An apparatus for manufacturing a switch,comprising: means for forming a first plate suspended adjacent to afirst substrate by at least one spring beam; means for coupling at leastone conductive bar to the first plate at one location on the at leastone conductive bar; means for coupling the at least one conductive barto a first contact which can protrude into without touching the firstplate, at another location on the at least one conductive bar; and meansfor configuring the first plate to activate the switch by pressing theat least one conductive bar coupled to the first plate against a secondcontact.